Read this one in the '90s.
The cider barrel on tap in a British pub seems to have run dry, despite not having been on tap very long and still weighing far more than an empty barrel should weigh. When it is opened, a body is found inside. The body turns out to be a Yank airman or soldier who had disappeared during WW II. (Not sure how many years after the war this is set; I think maybe in the '50s.) The Yank had had some connexion the daughter of the farmer who had made the cider in that barrel.
The Body in the Barrel is a hoary old urban legend. You can find it in Jan Harold Brunvand’s books, and the Paradox Press Big Book of Urban Legends. Usually it’s the body of a famous person being transported by ship – i don’t think I’ve heard the variation of the Yank in the Cider Barel before, but it’s basically the same story.
Don’t bother looking for it in the papers – it almost certainly ain’t true.
Although NBC News reported something similar as an actual news story. I suspect they were just taken in, though
Nothing to do with the book I’m looking for.
Sounds like the book is based on the legend, though.
Could well be, but it was a good book and I want to read it again.
I was actually going to ask about a different book (also a mystery), but I happened to think of this one as I was starting to type the OP and now I can’t remember what the other one was. 
I apologize if I derailed your Train of Thought.
Not your fault - it was already derailed by the time I finished typing the OP. It will come back to me later. When I’m nowhere near the computer, no doubt. 
Aha – I have remembered what the other mysterious mystery was!
Read this one in the late '90s or early '00s.
Skeletal remains are found buried in a field in Maryland(?). The skull is turned over to a forensic sculptor, who applies strips of clay in the right amounts in the right places to re-create the man’s face. After finishing, she steps back to look at her work – and discovers that she has just completed a bust of JFK. The rest of the book, involves running from the bad guys whilst trying to figure out how JFK got buried in somebody’s field…